1. Field of the Invention
The present invention relates to conductive pastes and ceramic electronic components. The present invention particularly relates to a conductive paste used to manufacture external conductors of ceramic electronic components and also relates to a ceramic electronic component such as a monolithic ceramic capacitor.
2. Description of the Related Art
In general, ceramic electronic components such as monolithic ceramic capacitors are prepared by a procedure in which a conductive paste for forming external conductors is applied onto both ends of ceramic elements and the resulting ceramic elements are dried and then fired. In order to enhance the solder wettability and the soldering heat resistance, the external conductors are coated with metal coatings such as Ni coatings, Sn coatings or solder coatings.
Examples of the conductive paste include a dispersion containing an organic vehicle; conductive powder, such as Ag powder, Ag—Pd powder or Cu powder dispersed therein; and glass powder (glass frit) dispersed therein. A known glass powder containing zinc borosilicate is disclosed in, for example, Japanese Unexamined Patent Application Publication Nos. 59-184511 and 6-349313.
Since zinc borosilicate glass has a low softening point in general, the glass melts and flows during firing. Therefore, there is a problem in that the molten glass extends to the surfaces of the external conductors to cover the electrode surfaces and this causes a deterioration in the platability of the external conductors. A deterioration in platability leads to inferior solder wettability and soldering heat resistance.
Since the molten glass leaks out of the external conductors to flow into the interfaces between the ceramic element and the external conductors, the external conductors become porous. Therefore, there is also a problem in that a plating solution penetrates the external conductors during plating and this causes a decrease in the adhesion between the ceramic element and the external conductors. Furthermore, there is a problem in that when the plating solution is present in the external conductors, water contained in the plating solution is vaporized and expanded by heat applied thereto during component-mounting so rapidly as to appear to cause an “explosion” in the external conductors, disturbing their integrity. The zinc borosilicate glass with a low softening point is soluble in the plating solution and this promotes the penetration of the plating solution.